1. Field of the Invention
The present invention relates generally to motor controllers, and more specifically to a motor controller for controlling the amount of force applied by a motor to move an object from an at-rest position.
2. Related All
Magnetic tape drives are commonplace in today's computer industry. These tape drives are used to store digital information onto magnetic tapes and to subsequently read the stored information. Two examples of a magnetic tape drive are the IBM 3480 tape drive available from International Business Machines in Armonk, N.Y. and the StorageTek 4480 tape drive available from Storage Technology Corporation in Louisville, Colo.
Magnetic tapes are typically available in two common formats: the cassette and the cartridge. The cassette tape is a two-reel mechanism that includes a supply reel and a take-up reel. Cassette tape drives thread the magnetic tape along a transport path, past one or more magnetic transducer heads, and then transport the tape such that it travels along the transport path. The tape is taken from the supply reel and wound onto the take-up reel. For tape drives capable of operating in a forward and reverse direction, the take-up reel and the supply reel exchange functions when the direction
For cartridge tapes, the take-up reel is external to the tape cartridge and is typically provided internal to the tape drive. When a cartridge is inserted into a tape drive, it is threaded along the transport path. The threading is accomplished as follows: an extractor arm "grasps" a leader block at the free end of the tape. The extractor arm pulls the tape by the leader block along the transport path past one or more transducer heads, idler arms, capstans, and the like. The tape is then fastened around the take-up reel. Once this is accomplished, the tape can be transported along the transport path and read/write operations can take place. A thread motor is used to drive the extractor arm. Depending on the tape drive utilized, the tape transport path may be an irregular path.
Thus, leader block transport can be discussed in terms of three phases. First is the initial phase of leader block extraction during which the leader block is pulled from an at-rest position such that the tape begins to spool from the supply reel. Second is the threading phase, during which the tape is threaded along the transport path. Finally, there is the transport phase, during which the tape is moved along the transport path across the tape heads.
Conventional tape transport systems are implemented such that the leader block is extracted by applying a pulse of current to the thread motor sufficient to overcome the static forces of friction. This pulse of current results in a constant acceleration profile versus time. The inventor has discovered that this conventional method "jerks" the leader block from the cartridge, thereby causing violent reactions in the tape. Problems encountered include stretched and ripped tape, tape oscillations resulting in normal forces along the tape, and tape overshoot.
What is needed is a tape threading mechanism that does not "jerk" the tape out of the cartridge housing during the extraction phase, but instead provides a smooth extraction. However, an additional challenge is that this smooth extraction must be performed in such a manner as to optimize the time required to thread the tape. Faster threading times reduce latencies associated with access to data stored on tape cartridges.